1. Field of the Invention
This invention relates to the preparation of metal bromides and more particularly to an improved process for preparing alkali and alkaline earth metal bromides by reacting a basic alkali or alkaline earth metal compound with bromine and a reducing agent in the presence of water.
2. Description of the Prior Art
The prior art has long taught the preparation of metallic bromides by reaction of a basic compound of an alkali or alkaline earth metal (e.g. sodium, potassium, calcium, and the like) with bromine in the presence of a reducing agent (e.g., urea, cyanamide, ammonium carbonate, ammonium bicarbonate, formamide, carbamates, ammonium cyanide and formic acid, oxalic acid, and their salts) which is converted to water and/or a gas.
More particularly, British specification No. 285,915 describes the preparation of calcium bromide by reacting a "non-acid" calcium compound (e.g. calcium oxide, hydroxide, and/or carbonate) with bromine in the presence of a reducing agent which is converted to gas and/or water. The reducing agent insures that substantially no bromates or hypobromites are formed as by-products. This patent describes several reaction sequences, among which are addition of the metal salt to a reaction medium comprising reducing agent, bromine, and water (Example I); addition of an aqueous solution of metal salt and reducing agent to an aqueous bromine reaction medium (Example II); and addition of bromine to a reaction medium comprising the metal salt, reducing agent, and water (Example (III).
U.S. Pat. Nos. 1,863,375 and 2,007,758 broadly relate to a process for preparing metal bromides employing ammonia to retard the formation of bromate and hypobromite. The U.S. Pat. No. 1,863,375 relates to the recirculation of an aqueous ammonia containing metal salt solution through a tower absorber in which it is exposed to bromine vapor. The U.S. Pat. No. 2,007,758, relates to the same general process, but is specifically concerned with means for recovering the spent ammonia evolved from the reaction mixture.
U.S. Pat. No. 2,269,733 describes the reaction of an alkali or alkaline metal compound with bromine in the presence of one of a variety of reducing agents. Several alternative reaction sequences are described in the patent. For example, at page 3 col. 2, lines 48-53, there is described the simultaneous addition of bromine and metal salt to a mother liquor, with an excess of reducing agent preferably being employed (see, page 3, col. 1, lines 58-62).
Alternatively, the patent also describes a two step process in which ammonia and bromine are first reacted in the presence of mother liquor to form ammonium bromide, with the metal salt thereafter being added together with additional bromine (see page 4, col. 2 line 3 through page 5, col. 1, line 66).
The foregoing processes have not, however, been entirely satisfactory. A difficulty experienced with all of the described prior art processes has been the problem of foaming. Foaming due to carbon dioxide evolution cannot be adequately controlled at acceptable productivity levels by the use of conventional antifoaming agents.
The foaming problem has been aggravated with prior art processes because carbon dioxide is not evolved in quantities proportional to the added bromine. Thus, when bromine is added to a reaction medium comprising the basic metal compound, reducing agent and water, carbon dioxide evolution does not begin until about one-third of the bromine has been added, with all of the carbon dioxide being evolved during the completion of bromine addition.
Furthermore, when the prior art reaction sequence of adding bromine to a basic aqueous reaction medium to which the metal salt and reducing agent have first been added, violent reaction conditions have been experienced at the point when the pH shifts from the basic to acidic, thereby demonstrating that significant safety risks are involved with the process.
Accordingly, an object of this invention is to provide a new process for preparing metal bromides which avoids the disadvantages experienced with prior art processes.
A further object is to provide a process for producing metal bromides by reacting alkali or alkaline earth metal salt with bromine in the presence of a reducing agent and water in which carbon dioxide is generated in quantities proportional to that of the added bromine.
A further object is to provide a process of the character described which can be practiced without creating safety risks.